Natural gas is a mixture of hydrocarbons and non-hydrocarbon gases found in geologic formations beneath the earth's surface, often in association with petroleum. As obtained from the ground, raw gas contains a number of impurities which must be removed at some point. The principal impurities in natural gas are water, carbon dioxide, hydrogen sulfide, organic sulfides and condensable hydrocarbons, such as propane, butane and pentane.
Generally speaking, hydrogen sulfide and organic sulfides (collectively here "sulfides"), because of their corrosiveness and toxicity, are removed in the field prior to introduction to a pipeline for transport to a market or off-site processing plant. The maximum level of hydrogen sulfide, "H.sub.2 S", the primary sulfide impurity of natural gas, permitted to be introduced into a pipeline is often limited to 0.25 grains per 100 cubic feet of gas, that is, 4 parts per million (ppm), although in some instances, up to 1 grain per 100 cubic feet (16 ppm) is acceptable.
H.sub.2 S, like hydrocarbon components of natural gas, exists in the gaseous state at normal temperatures and pressures. There is a variety of approaches to removing H.sub.2 S from, or at least substantially reducing the amount of H.sub.2 S in natural gas, i.e., "sweetening" natural gas. One general approach is to expose the raw natural gas to a treatment liquid containing an agent which chemically reacts with H.sub.2 S, a so-called H.sub.2 S scavenger. Usually, the natural gas is bubbled through the liquid and as the bubbles rise through the liquid, H.sub.2 S comes into contact with the agent to react and form a non-gaseous, or relatively non-volatile, product. The H.sub.2 S thus becomes trapped in the liquid and is removed from the gas.
One type of agent often used to react with H.sub.2 S is the reaction product of an organic amine compound having an "active hydrogen" and an aldehyde. An active hydrogen herein is a hydrogen directly bonded to a nitrogen atom. It is this type of scavenging or trapping agent to which the present invention relates. Examples of such agents are described in U.S. Pat. No. 4,978,512 (issued Dec. 18, 1990 to Dillon) and U.S. Pat. No. 5,462,721 (issued Oct. 31, 1995 to Pounds et al.). The specifications of both of these earlier patents are incorporated herein by reference.
As described by Pounds et al., there is a number of characteristics that are considered desirable in treatments containing such active agents. Generally speaking, it is desirable that the agent be highly reactive, i.e., the H.sub.2 S (or organic sulfides) should react readily with the agent as the H.sub.2 S passes through the treatment liquid. At the same time, it is desirable that the agent remain effective even in the presence of a large excess of CO.sub.2, that is, it should be selective. The scavenging agent should be easy to handle, that is, be of appropriate viscosity, have a suitable pour point and low toxicity--it should have no free formaldehyde nor produce a vapor containing formaldehyde. In terms of performance, it is also desirable that the agent not foam excessively in use.
It is also desirable for the reaction product(s) of the H.sub.2 S and the scavenging agent to have certain characteristics. It has generally been the approach to use single phase treatments. In particular, liquid treatments in which the products of the H.sub.2 S trapping reaction are soluble in the liquid have thus been considered highly desirable because of the ease of disposal of the spent fluids.